Memory of Survivors: A Seedborne Endophyte Mediates the Impact of Severe Drought on Plant Maternal Fitness and Progeny

ABSTRACT Drought events can have long‐lasting effects on plant performance and progeny traits. We investigated how an early, severe drought at the seedling stage affected plant fitness and seed traits in Lolium multiflorum , and whether these responses were modulated by symbiosis with the vertically transmitted fungal endophyte Epichloë occultans . Drought caused significant mortality, and the symbiosis with the endophyte improved survival independently of plant biomass. Surviving plants fully recovered aboveground biomass and seed production only in the presence of the endophyte. Isotopic analyses indicated that only non‐symbiotic plants showed reduced stomatal conductance during seed set, which likely explains their lower seed production. Seeds from drought‐exposed symbiotic plants had higher concentrations of compatible solutes (mannitol and sorbitol) and starch. However, symbiotic seeds from drought‐exposed plants showed reduced germination under intermediate water potential. This response was associated with a drought‐induced increase in the base water potential (Ψ b ). Alternatively, constant hydrotime was positively associated with starch content. Our results suggest that endophyte symbiosis enables recovery from early drought via osmotic adjustment and photosynthetic maintenance, with intergenerational responses mediated by changes in seed biochemical composition and germination. These findings highlight the role of vertically transmitted endophytes in plant memory of stress and drought resilience across generations.